This is what I've heard.....Burst main on the first floor, above Thames and loads of equipment rooms.
The water was seen pouring out of a ground level door into the car park. When it first happened the fire alarm went off, first 'Prepare to evacuate'. The fire service rang the tower shortly afterwards to check if it was a test or a real alarm. They said they already had two people in the subway under the tower which leads from the Tube station to T2 investigating flooding there (subsequently found to be water coming from the tower building).
Lots of tels equipment ruined, including the Code-Callsign conversion thingimy-wotsit. Not sure how widespread that was, but I know Manchester rang up and asked why they were only seeing squawks!
Apparently the smell in the tower was very very rank when the night shift were going home, lots of soggy carpets, walls and ceilings, I've heard the ceiling tiles were actually dissolving in the water.
Gonzo.